1. Field of the Invention
The present invention relates to magnetic recording media such as video tapes, audio tapes, memory tapes, and the like.
2. Description of the Prior Art
Recording media such as tapes which are designed to be thin and where the speed of feeding the tape is designed to be slow, such as high density compact video tapes, are particularly effective due to the fact that the coefficient of friction on the base surface can be reduced without a broad reduction in the Young's modulus of the magnetic layers occurring.
The surface of the support opposite the magnetic layer in a magnetic tape can be divided into the following classes:
(1) TAPES IN WHICH THE SUPPORT SURFACE IS EXPOSED;
(2) TAPES IN WHICH A BACK COATING LAYER OF CARBON BLACK IS PROVIDED ON THE BACK SUPPORT SURFACE; AND
(3) TAPES IN WHICH A BACK COATING LAYER OF A SOLID LUBRICANT SUCH AS GRAPHITE, MOLYBDENUM DISULFIDE, ETC. IS PROVIDED ON THE BACK SUPPORT SURFACE.
Of these, tapes of type (3) above have the lowest coefficient of friction. However, even with tapes of type (3) above,
(A) the solid lubricants described above are scraped off by repeated use thereof to thereby cause drop out (of the signals) of the magnetic tapes; and
(B) the surface roughness of the back coating is transferred (printed through) onto the surface of the magnetic layer during storage of the magnetic tape, and particularly in the case of a high density tape, a serious deterioration in the high frequency signals as well as a serious increase in noise occur.
With tapes of type (2); the following technical problems are encountered:
(C) the coefficient of friction is not satisfactory since it is higher than in tapes of the type (3) while lower than in tapes of type (1);
(D) the same problem as in (B) above is encountered; and with tapes of type (1)
(E) the coefficient of friction is high so that scratching tends to occur and hence, scraped dust of the base due to the scratching adheres to the magnetic recording layer to thereby cause drop out to occur.
Of these problems, (B) and (D) most adversely affect high density tapes. The following methods are known to improve (A) above:
(A') a method in which a base support having a low surface roughness at the surface adjacent a magnetic recording layer and having a high surface roughness at the surface opposite the magnetic recording layer is employed;
(A") a method in which a lubricant is coated on the surface opposite the magnetic recording layer during the preparation of the base or the preparation of the magnetic tape; etc. However, even with these methods, the following disadvantages are still encountered:
in (A'), this kind of base support has not been popularized yet in general and hence the cost is expensive; and
in (A"), a reduction in the coefficient of friction is effectively facilitated immediately after coating, but the effect in the reduction of the coefficient of friction disappears with the passage of time; etc.
In addition, tapes of types (2) and (3) mentioned above have the following disadvantages, as compared to the tapes of type (1);
(a) due to the presence of a backing layer, extra expense for raw materials as well as preparation of a coating solution thereof are incurred; and
(b) due to the double coatings required to coat a magnetic recording layer and to coat a backing layer, the yield rate is naturally decreased as compared to that of the single coating of a magnetic recording layer; etc. Further, use of method (A") has the disadvantage of (a) above.
On the other hand, a variety of methods of incorporating lubricants such as higher fatty acids, higher fatty acid esters, paraffin type hydrocarbons, silicone oils (e.g., dimethyl polysiloxane, diphenyl polysiloxane, etc.), or the like into magnetic recording layers have been studied and examined. In particular, silicone oils are preferably employed as lubricants due to the good results obtained.
The above-described additives are disclosed in Japanese Patent Publication Nos. 18064/66, 186/68, 669/68, 15624/72, 14249/74, and 14252/74; Japanese Patent Application (OPI) Nos. 32904/75, 99701/75, 13210/75 and 40903/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".); U.S. Pat. Nos. 2,654,681, 3,470,021, 3,497,411, 3,525,694, and 3,634,253, etc.
However, sufficient lubrication and durability cannot be successfully achieved for the back surface of the support of video tapes using these conventional additives.
Further, when coated as a backing layer as in method (A") above, only a trace of the lubricants ultimately remains on the surface of the base support of a tape after it has been passed through the various steps subsequent to coating. Furthermore, even the trace of lubricants is transferred into a magnetic layer with the passage of time if the tape is allowed to stand so that the coefficient of friction of the base surface increases. Even when tapes are repeatedly used in a VTR, the coefficient of friction also increases.
To the contrary, if the lubricant in accordance with the present invention is incorporated into a magnetic layer, a low constant coefficient of friction is always maintained. The reason for this is believed because, if the lubricant exuded from the magnetic recording layer is once adhered to the base surface, an equilibrium state between exudation and removal of lubricant is maintained even if allowed to stand for a long period of time. In addition, it is also assumed because, even if the amount of lubricant on the base surface and the surface of the magnetic recording layer is temporarily reduced or removed by running tapes in a VTR, the lubricant exudes from the magnetic recording layer immediately when the tapes are wound up, and adheres to the base surface.
Conventionally known lubricants which are generally incorporated in magnetic layers have little effect of decreasing the coefficient of friction of the base surface. In addition, in order to enhance the effect, a large amount of lubricants should be incorporated into the magnetic recording layers, but this is not preferred because a reduction in the elasticity of the magnetic recording layers occurs.
On the other hand, higher density recording is desired for magnetic recording tapes, particularly for video tapes, and due to improvements in the efficiencies of magnetic recording layers, recording wavelengths have been changed from 6 .mu.m to 1 .mu.m, and the running speed of tapes has been reduced from 38 cm/sec. to 3.33 cm/sec. In addition, due to high density recording signal requirements, any changes which occur due to tension, temperature, friction, etc. cannot be ignored.
For example, comparing a tape with a running speed of 38 cm/sec. and a recording wavelength of 4 .mu.m with a tape with a running speed of 3.33 cm/sec and a recording wavelength of 1 .mu.m,
(i) the surface property of the latter tape should be improved, so that "stick-slip" of the tape tends to occur; and
(ii) an uneveness in the running of the latter tape tends to occur.
On the other hand, now the tendency is to convert high density compact video tapes into cassette tapes. With the conversion of cassettes into smaller-sized cassettes and of VTR tapes to a small size, an extremely marked characteristic is to reduce the thickness of the tape. As a result, the mechanical strength of the tape is weak so that the shape of the tapes tends to change and the tape has poor quality. The mechanical strength of a tape is proportional to the cube of the Young's modulus. A base having a high Young's modulus, e.g., an Al-deposited base, can be used, but this approach has not been employed practically yet due to economic considerations and other problems. As a result, reduction in the Young's modulus of the magnetic recording layer is not desired.